Mysterious elevated chloride levels documented in Des Moines streams

Elevated levels of chloride have been consistently documented in at least 11 metro area stream test sites, according to a new report from the Polk County Conservation Board.

Why it matters: Excessive amounts of the naturally occurring element can be toxic to some aquatic life and could make stream water, which feeds into DSM drinking water sources, taste salty.

• A planned $117 million water trails project that runs through downtown Des Moines is adding to the urgency and interest in addressing the issue.
• While chloride is not usually harmful to human health, it could negatively impact overarching conservation and recreational goals by diminishing the natural habitat that attracts trail users.

Details: The problem sites are primarily on the city's south side and in West Des Moines, Polk County Conservation Board's monitoring program shows.

• Most of the streams drain into the Des Moines and Raccoon Rivers — where we get our drinking water. But the rivers are well diluted and have far lower levels of chloride.

By the numbers: Areas with a five-year average of greater than 100 milligrams per liter are considered to have elevated levels in the Polk County review.

• Public drinking water standards allow for up to 2 ½ times that amount, which no site average exceeded.
• Yes, but: The EPA recommends sodium levels not exceed 30-60mg per liter to avoid any hint of salty water.

What's happening: Conservationists don’t yet know what’s behind the spikes, Polk County Conservation director Rich Leopold told Axios. Their working theories:

• It’s not likely from de-icing chemicals or salt runoff. The spikes are relatively consistent, even in warmer months when roads go untreated.
• Sewer overflow or breaks in pipes carrying wastewater could be to blame. Some of Des Moines’ underground infrastructure is more than 100 years old.

What's next: Leopold tells us that scientists and conservationists will spend the next few months analyzing the data to see whether it can help pinpoint the reason for the spikes.

This story first appeared in the Axios Des Moines newsletter, designed to help readers get smarter, faster on the most consequential news unfolding in their own backyard.

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